In days gone by you could take the last train out of London and doze in the hall before the 300, but i can see why the org decided not to offer that this time.
I must say it was surprised at how easy it was to enter the event then bung on a 50k ECE, matter of minutes and one website for each. (Probably turn out to be a damned sight easier than riding the thing...)
Depending on where you live you might be able to do an ECE riding down from London and entering say, the 50k or the 100k? For instance, Hyde park corner is 80k says Google maps, so you could ride there, do the 50k then ride home for a 200k. (If you were insane enough to ride across that part of London, that is.)
But you get my drift - with such a wide range of distances in one event your options are very wide. In fact, if you lived 75k away you could ride an ECE there and back with the 300k and make it a 600k.